1. Field of the Invention
The present invention relates generally to spread spectrum systems and more specifically to a marine navigation receiver well adapted for use with the Global Positioning System.
2. Description of the Prior Art
The Global Positioning System, which hereinafter will simply be referred to as GPS, is a developmental satellite navigation system. When fully deployed, GPS will consist of a constellation of 18 satellites which will enable a user to view no fewer than four (with slight exception) useable satellites from any location at any time. One suggested configuration has the satellites placed in six earth centered circular orbits of three satellites, each orbital plane being inclined at an angle of 55 degrees to the plane of the equator, and the six orbital planes being spaced at 60 degrees along the equator. The satellites orbit at approximately 11,000 nautical miles above the earth and are maintained in an almost perfect circular orbit by the ground control segment of the GPS system. Each satellite transmits data which enables the user to calculate a correction for its clock time and to calculate its position to within several meters. The satellite clock and orbit are monitored by ground control stations and the data is uploaded to each satellite every day for a 24 hour period. The data transmitted by the satellite is changed hourly to preserve the desired system accuracy. Coarse ephemeral data for all satellites is transmitted by each satellite in the system on a sequential basis, enabling the user to create an almanac for the entire system from any one signal.
The data is modulated by two pseudorandom noise codes resulting in a spread spectrum transmission from the satellite. This technique provides the ability to select any one satellite by the use of a family of codes which have very low cross correlation coefficients with other members of the family. These codes also possess very low auto-correlation sidelobe responses allowing the unambiguous measurement of time delay and hence range. The correlation or "despreading" process in the receiver provides a large "processing gain" which provides good signal to noise ratios with low transmitted signal levels, and tends to reject interfering signals which are usually found in the marine environment. The two codes, "clear/acquisition" (C/A code) and "precision" (P code), modulate the carrier in a quadriphase manner so that each may be received individually without interference by or even the need to demodulate the other. As the name implies, the C/A code is transmitted in the clear and is used for the initial acquistion of each satellite. The P code, which is presently transmitted in the clear but which may be encrypted at the discretion of the system for national security reasons, provides the highest accuracy of range measurement and hence system accuracy.
One of the larger contributors to system error, if not corrected, would be the variable ionospheric delay of the carrier as a function of night/day, sunspot activity, geographic location, etc. Fortunately, this delay, though variable, is a well behaved function of frequency being proportional to 1/F.sup.2. Therefore, the signal is transmitted on two carriers, the primary (L1) carrier at 1575.42 MHz and the secondary (L2) at 1227.6 MHz enabling the user to compensate for the ionospheric delay. An approximation of the delay correction is transmitted in the satellite data, allowing the omission of the second frequency while still permitting correction of about 50% of the error.
If the clock in the receiver is exactly aligned with GPS time, then the receiver can measure directly the range to each satellite in view. If the receiver's location contains three unknowns (latitude, longitude and altitude), then three independent range measurements are necessary for the solution of position. If the receiver clock is not in exact alignment with GPS time, then an additional unknown is entered into the equations for solution and an additional independent range measurement must be taken to allow solution with four unknowns.
If the receiver is located on a vehicle of high dynamic performance, then the vehicle control system requires position information at a rate that dictates the simultaneous reception and measurement of the signals from four satellites. In practice, a fifth channel would be added to expedite the changing of satellite selections. In extreme situations, such as might be found in military applications, it is also desirable to couple the vehicle's inertial navigation system into the receiver's tracking system to aid in "coasting" during periods of temporary signal loss due to aircraft attitude, enemy jamming, etc.
The requirements for a receiver to achieve maximum system accuracy and capability include: five simultaneous receiver channels, dual frequency operation (L1 and L2), dual code operation (C/A and P), and inertial aiding input. For many applications, the receiver requirements may be quite different; some have high accuracy, low dynamic requirements, including remaining in a fixed location for some time (military land vehicles); some have relaxed accuracy requirements but modest dynamic behavior (light aircraft); some require accuracy beyond the stated system capabilities which may possibly be obtained by special receiver techniques (survey and time transfer receivers); and some have minimal requirements of accuracy and dynamic capability (marine navigation).
Using the U.S. Coast Guard's requirements for coastal confluence navigation, the accuracy requirement is set at: .+-.1/4 nautical mile, 95% probability; and the dynamic capability at a system data update each 60 seconds. These specifications can be met with a receiving equipment having: single frequency receiver; C/A code only; one channel, sequenced between four satellites; and latitude, longitude and time outputs. Several existing navigation systems provide, to some degree, the information expected of a GPS system. The cost of these existing systems, however, is relatively high. Accordingly, it can be appreciated that there is a need for a relatively low cost receiver which is compatible with GPS and which can fulfill marine navigation requirements.